Geology. - Geology of Central~Curaçao. By L. W. J. VERMUNT and M . G . RUTTEN. (Communicated by Prof. L. RUTTEN. )

(Communicated at the meeting of February 28. 1931.)

In the spring of 1930 some students of geology visited Curaçao. Aruba and Bonaire in the company of Prof. L. RUTTEN.

It is intended to give here a report of the result of their investigations in Central~Curaçao.

The island of Curaçao. which had already been investigated cursorily by K. MARTIN. was at a later date treated more thoroughly by J. J. H. MOLENGRAAFF and described in "Geologie en Geohydrologie van het eiland Curaçao" 1929. In said publication the geology of the western part of C~ntral~Curaçao is only outlined roughly; consequently there was every reason to subject this region to a more minute investigation. the more. as the possibility existed of finding in this district indications about the pre-or old~tertiary trends in the island.

Stratigraphy of the "Midden~Curaçao" beds. In this district the "Midden~Curaçao" beds have their greater ex tent

and nowhere in the island are they better developed. We can divide them into three zones viz. an oldest conglomerate zone. contrasting sharply with the middle zone. consisting of sandstone and shale and a third pure1y shaly top zone.

Clastic constituents of the conglomerate zone. On examining a number of slides of conglomerates under the microscope

we find. as is to be expected. common constituants. The cement in all the conglomerates is calcareous. but present in quantities which vary conside~ rably. The following constituants occur in practically all rocks:

1. Diabase varying from more or less weathered to fresh. A few diabase~like rocks also contain much epidote; some of them are entirely epidotized. A fragment which may be allied to these rocks is a pebble of a quartz-epidote rock. (D 11918.) 1)

2. Rounded pieces of chert consisting of fine quartz-aggregates. frequently very dear. occasionally greatly polluted. bearing dear evidence of stratification.

3. Large idiomorphic and rounded feldspars. usually albite.

I) The "D" numbers refer to the collectIon of microscopical slides in the Geol. Inst. of the Utrecht university.

18 Proceedin9s Royal Acad. Amsterdam. Vol. XXXIV. 1931.

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4. Rounded and angular quartz-crystals. frequently cataclastic. some­times giving the impression of being of effusive origin.

5. Coarse quartzitic quartz-aggregates. At the si de of these constituents of frequent occurrence. a number of

interesting. rare elements are also met with in the conglomerate strata. 6. Amphibolites. consisting of hornblende (pleochroism: yellowish

green to bluish green). between which dusty acid plagioclase (C 64) 1). while in one thin-section quartz-drops occur in the feldspar (C 44).

7. Mica-schists. composed of quartz and sericite (C 58. C 59). 8. Microcline gneiss. a grain consisting of a microcline crystal and a

cataclastic quartz (C 43). 9. A grain of a pegmatitic rock. consisting of very dusty acid

plagioclaSe and quartz (C 28. 0 11918). In many preparations we find numerous transverse sections of organisms:

Lithothamnium. small gastropodes. lamellibranchiates. siphonal calcareous algae and foraminifera (Nodosaria. Textularia. Globigerina).

Clastic constituants of the sandstone and shale zone of the "Midden­Curaçao" strata.

The sandstones contain a great amount of mica and chiefly angular and rounded quartz. which is in a few cases of effusive origin with glass enclosures and gasbubbles. The mica mainly consists of small muscovite plates. while a little biotite sometimes occurs. In these deposits. too. we co me across a fair number of rounded chert grains.

In slide 0 11927 we find a few grains of a chlorite-mica-schist. consisting of sericite and chlorite. between which there is fairly much quartz. There also occurs a muscovite-biotite-gneiss. composed of weIl trended muscovite­scales. a pale green decolorized biotite and feldspar. which cannot be determined exactly. It should still be mentioned that apatite. zircon and tourmaline occur in slide 0 11986. In some places the sandstones are exceedingly calcareous ; occasionally they alternate with limestones.

Shale-zone.

In the shale-zone. 300 metres north of the country house Rif. there is a thin sill; under the microscope the rock proves to be an albitite. consisting of smaller and bigger albite-oligoclase-crystals. secundary quartz and calcite and a little chlorite. In all probability we have to deal with a vein-rock genetically connected with the quartz-diorites met with in North­Curaçao. MOLENGRAAFF only found eruptive dykes in the diabase and the "Knip" strata. Their occurrence in the "Midden Curaçao" strata points to the fact that the intrusions are younger than these beds and the Lower­Eocene age of . these eruptive rocks. assumed by MOLENGRAAFF becomes more probable.

I) The "C" numbers refer to our collection of Curaçao-rocks.

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Nature and origin of the clastic material of the " Midden Curaçao" strata. It is dear that at the contact of the "Midden~Curaçao " conglomerate

and the older strata we may ex peet an increase of dastic material from these ol der strata. Consequently we Eind. for instanee. where the road from Klein~Fontein to Sabana Meimei crosses the boundary Diabase "Midden Curaçao" beds. a greatly predominating quantity of diabase pebbies in the conglomerate. in fact to su eh an extent that the weathered conglomerate can hardly be distinguished from weathered diabase. South of the Seroe Rondo there is an outerop of "Seroe Teint je" limestone and here. too. we observe in the immediate vicinity a great increase in the number of Iimestone pebbles in the conglomerate.

As minerals of the Conglomerate~zone we mentioned chiefly quartz and feldspar. Neither the dusty. frequently catadastic quartzes. nor the big dear quartzes with an effusive character can be derived from any Curaçao rock. In N .W . Curaçao a number of quartz~bearing rocks occur as quartz~ diorite and quartz~augite-diorite a .s.o. which have been described by MOLENGRAAFF. but the distribution of these rocks is so slight. that their share in the composition of the "Midden~Curaçao" strata may be neglected. Besides. MOLENGRAAFF ascribes a Lower~Eocene age to the quartz~

dioritic rocks and consequently considers them as being younger than the "Midden~Curaçao" strata.

The rounded grains of quartzitic rocks. too. cannot have originated from Curaçao.

As to the feldspars. it is not impossible that they have originated from the diabase.

The rounded chert fragments can also easily be recognised as material from the underlying ol der "Knip" strata.

The muscovite. especially so numerous in the sandstones and the rare biotites must have been conveyed there from outside Curaçao.

Quite alien to the underground of Curaçao are the amphibolites. mica­schists. pegmatites and microdine~gneiss from the conglomerate-zone. a chiorite~mica~schist and a muscovite~biotite~gneiss from the sandstone~

shale~zone . The comparative scarceness of these rocks and the smallness of the granules point to a great distance of the place of origin. in contrast with Bonaire. wh ere in the "Soebi Blanco" formation we find large pebbies of gneiss material predominant 1).

From this it appears. consequently. that though the "Soebi Blanco" strata in Bonaire and the "Midden Curaçao" strata were deposited in about the same period (Iatest Cretaceous or oldest Tertiary) the circumstances must have been quite different for the two islands.

As the onIy district in the vicinity wh ere we come across such schistose and gneissose rocks is in N. Venezuela. it is not improbable that the

1) P. PIJPERS. The occurrence of foreign pebbles on the island of Bonaire. These Proceedings 34. 1931. p. 168-174.

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rocks alien to Curaçao have originated frome there or from a territory geologically allied to the N. Venezuelan mountains, which was situated between Curaçao and South~America in pre~upper~cretaceous times.

Thickness and distribution of the "Midden~Curaçao" beds. The attached geological map shows that the conglomerate zone is missing

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in the West and consequently here the sandstone~shale~zone lies immediatelyon the "Knip" beds.

One kilometre to the East there is a distinct fault . Here the conglomerate zone commences, which is approximately 200 metres thick on the outset.

Further to the East the conglomerate-zone gets much thicker and Sou th­East of Cas Abau (Section 3) it is about 450 metres.

North~East of the country house Porto Marie the "Midden~Curaçao" beds curve to the N.N.E.; the conglomerate zone gets broader and broader, while the pebbles get bigger and bigger. Where the anticline clips to the E.S.E. an estimate of the thickness of the conglomerates in the longitudinal profile along the axis is possible : the thickness here amounts

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to approximately 550 metres. In the N.E. the conglomerates He immediately on the diabase. Consequently we must assume here. that the "Knip" beds were removed already before the deposit of the "Midden~Curaçao" ... beds. So this. together with the numerousness of large "Knip" pebbles in the conglomerate. points to an unconformity between "Knip" and "Midden~ Curaçao".

The boundary between the conglomerate~zone and the sandstone~

shale~zone is very distinct. In general it is in conformanee with the boundary conglomerate~zone - "Knip" beds. As a result of strong folding. e.g. near St. Willebrordus and South of Jan Kok. it is less possible to give definite figures for the thickness of the sandstone~shale~zone than for that of the conglomerate zone.

We may however assume. that its thickness is at least about 700 metres. (Section 5.) Nothing can be said about the thickness of the shale~zone.

"Knip"~beds.

To the "Knip"~beds belongs the district surrounding the Seroe Mameter. We find in several places. f.i. South~East of the Seroe Mameter on the boundary with the "Midden~Curaçao" conglomerate "Knip"~chert which is quite caracteristic. In generaI. however. the "Knip" which we meet with on the Seroe Mameter. is developed atypicaIly; side by side with the normal chert and tuffaceous beds. we find here a calcareous~marlaceous facies. MOLENGRAAFF knew from the "Knip" dolomitized and calcified chert; here we have to do however with pure limestones. in which some­times very smaIl angular plagioclase and quartz~crystals occur. North West of the Seroe Mameter in the direction of St. Jan the quantity of chert increases. while in the same region the boundary between the sandstone~shale~zone of the "Midden~Curaçao" beds and the "Knip" beds is difficult to trace in consequence of the shaly development of the "Knip" beds and the diappearance of the "Midden-Curaçao" conglomerates.

The isolated outerop of "Knip" in the diabase~region South of Sabana·· Meimei is very remarkable. The stratified rock proves to be a fine vitric tuff under the microscope. We may probably assume this to be a smaII downfold in the diabase.

Tectonics of Central~Curaçao. On the geological map of the surroundings of Porto Marie the attention

is immediately attracted by an anticline distinctly directed East~South·­East and sharply dipping in that direction. consisting of "Knip" beds and "Midden Curaçao" beds. running from St. Jan tiIl past the Seroe Mameter. Said anticline. which is partly overturned on the Northern and Southern wing (section 111). still shows the same strike as MOLENGRAAFF gives for the great anticlinorium in North~West Curaçao and consequently proves that a bending of the main strike in an East-Western direction. as is assumed by MOLENGRAAFF already to occur to the North of this region.

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has not yet taken place here. If we assume this bending. to which points e.g. the direction of the boundary between conglomerate~zone and sand~

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stone~shale~zone East of St. Bastiaan. then it does not take place until much more to the South than is given by MOLENGRAAFF.

The whole series of "Knip" and "Midden~Curaçao" beds may. however. also be considered as being an East~South~East dipping anticlinorium; to th is might point e.g. a few strikes South of Koenoekoe Abau (a littlc East of the map). which run North~North~East. In this case the trend of the Old Tertiairy folds would therefore correspond more or less with that of the younger folds . The solution to this problem would have to be found from the trends in the Southern sediment belt of the eastern diabase region.

This is. however. no longer present in Curaçao. Consequently we have to take both possibilities into account.